The use of communication systems having wireless mobile communication units (MCUs) has become widespread. Wireless communication systems that operate within a cell or coverage area provide several important advantages over conventional wired systems. For example, wireless communication users can communicate in locations where wired service is not available or feasible, such as remote or rugged locations. Additionally, wireless communication users within the cell have much more mobility because the units do not have to be connected to a fixed wired network. These and other favorable characteristics make wireless communications ideally suited for personal, business, military, search and rescue, law enforcement, water vehicle, and other field related applications.
In some applications, it is desirable to maintain consistent periodic communication with one or more other MCUs. Such consistent communications are useful in situations that have a risk of incapacitation of the user, such as battlefield applications, recreational activities in rugged environments, or civilian search and rescue operations. If the periodic communications from the user cease, a search can be carried out at the last reported location. A variation on this theme could include an automated communications unit that periodically broadcasts a signal to other communication units, such that the signal itself can be employed to locate the user.
If an area is equipped with robust communications infrastructures (e.g., cell towers), there are relatively easy ways to establish and maintain such wireless communications to mobile users, by making use of the infrastructure. However, there are some endeavors, such as search and rescue and/or military applications, where it is necessary to provide such a wireless communications capability even in an area that does not have such a communications infrastructure, and in such cases it is much more difficult to establish and maintain such wireless communications to mobile users.
Mobile users can experience rapid variations in connectivity between node pairs (e.g., MCU to MCU, MCU to cell tower). Connectivity variations are primarily due to propagation loss variations. An individual connection suddenly breaks or reforms respectively as the signal-to noise drops below or rises above a connectivity threshold. Propagation loss and signal-to-noise ratio can vary quickly, particularly for fast moving nodes in complex terrain or in urban areas. Due to the complex relationship between terrain and propagation loss, it is difficult for individual mobile users to anticipate connectivity breaks. Thus, a mobile user may suddenly find his node disconnected from the network with little or no warning. Also, a disconnected mobile user may not know where to relocate to restore a network connection.